Preparation of mixed matrix composite membrane for hydrogen purification by incorporating ZIF-8 nanoparticles modified with tannic acid
Introduction
For its high calorific value and no pollution in combustion process, hydrogen has been considered as a kind of clean energy with great development prospects [1], [2], [3], [4]. In industry, hydrogen is produced mainly by water-gas shift reaction with the reaction products of H2 and CO2. Technologies for H2 purification include pressure swing adsorption, chemical absorption and membrane separation [5], [6], [7], [8]. Membrane separation technology has attracted more and more attention due to its advantages of less fixed investment, higher energy efficiency and simple operation [9], [10], [11]. Compared to H2-selective membrane [12], using CO2-selective membrane [13] for H2 purification is expected to be more efficient since the membrane area required for separation process is smaller and the hydrogen products are at high pressure, which is convenient for subsequent use [2], [14]. As a type of asymmetric membrane, composite membrane consists of a selective skin layer for gas separation and a non-selective porous supporting layer, which provides mechanical strength. Because of less requirement in material cost and mechanical strength for separation layer, composite membrane is a preferable membrane structure. To fabricate a composite membrane with high CO2 permeance and high CO2/H2 selectivity, it is necessary to optimize both the material for separation layer and the method of membrane fabrication.
On the one hand, the material used for separation layer should be of high CO2 permeability and CO2/H2 selectivity. Since the critical temperature of CO2 is higher than that of H2, the reactivity of CO2 is stronger than that of H2, and the molecular dynamics diameter of CO2 is larger than that of H2, it is necessary to enhance the solution selectivity and reaction selectivity, and weaken the diffusion selectivity in order to improve the CO2/H2 selectivity. Reaction selectivity is achieved when one component of the mixture reacts reversibly with the carrier in the membrane and the other components do not react [15]. Mixed matrix membrane material is a good choice because it can combine the advantages of polymer matrix and porous filler. Existing polymer materials that preferentially permeate CO2 mainly include some rubber polymers such as PDMS [16], [17], [18], [19], Pebax [20], [21], and a few glass polymers such as poly[1-(trimethylsilyl)-1-propyne] (PTMSP) [3], polymers of intrinsic microporosity (PIMs) [22] and PVAm [23]. Among these polymer materials, most of them achieve preferential CO2 permeation only by solution selectivity, while PVAm has both solubility and reaction selectivity for CO2/H2. Porous fillers applied in mixed matrix membranes include zeolites [24], metal organic frameworks (MOFs) [25], [26], covalent organic frameworks (COFs) [27] and so on. As a type of MOF, ZIF-8 has been widely used in the research of mixed matrix membranes due to its good hydrothermal stability, acid and alkali resistance, and it has been successfully commercialized [28]. The main problem in the field of mixed matrix membrane is the compatibility between porous fillers and polymer matrix. There are mainly two effective solutions for this problem including enhancing the physical interaction between particles and polymers, such as electrostatic attraction [29], hydrogen bond [30], and chemical bridging between particles and polymers [31]. Polymer modified ZIF-8 particles is one of the methods to weaken the interface defects of ZIF-8 fillers and matrix in composite membranes. Zhu et al. [32] prepared thin-film nanocomposite (TFN) membranes containing ZIF-8 (mZIF) hydrophilic modified by poly(sodium 4-styrenesulfonate) (PSS) in a polyamide (PA) layer. ZIF-8 modified with water-soluble polymer can be dispersed well in aqueous solution to avoid the avoid particle agglomeration, resulting in a notable increase of the water permeability with the retention rate unchanged. In Wang et al.’s research [30], ZIF-8 nanoparticles were modified by polydopamine (PD) coating, and Gao et al. [33] synthesized PEI-g-ZIF-8 in situ by adding hyperbranched PEI into Zn2+ and 2-methylimidazole methanol solution through a rapid stirring at room-temperature. Both obtained high-permselectivity MMMs, because the interfacial compatibility between fillers and the polymer matrix was enhanced through the hydrogen bond interaction formed by polymer (PD, PEI) grafted in ZIF-8 and matrix.
On the other hand, it is necessary that high-performance membrane materials can form thin and defect-free separation layer onto porous support through appropriate membrane preparation process, where pore penetration of the separation layer needs to be suppressed and good compatibility between the separation layer and the support should be ensured. The introduction of high-permeance gutter layer between separation layer and porous support is a simple and effective method to inhibit the pore penetration of separation layer. At present, the main materials used for gutter layer are PDMS, PTMSP [34], [35]. Because of its low price and stable performance, PDMS is the most widely used gutter-layer material, which has been reported to fabricate superhydrophobic membrane surfaces [36], [37]. However, at present, most of the membrane materials that preferentially permeate CO2 are hydrophilic or alcoholic, which are incompatible with these hydrophobic gutter-layer materials. To solve this problem, some researchers had modified the surface of PDMS to turn it into hydrophilic, such as depositing a layer of polydopamine on PDMS [38], surface grafting [16]. In 2013, Ejima et al. reported that the reaction between TA, a natural macromolecule polyphenol, and ferric trichloride (FeCl3) could form complexes which can attach to various surfaces [39]. The quinones formed by oxidation of phenolic hydroxyl in tannic acid could rapidly complex with Fe3+ and adhere to various surfaces in a short time [39], [40], [41]. The reaction between TA and Fe3+ have been applied in reverse osmosis and nanofiltration membrane research, which we believe can also be used for the hydrophilic modification of PDMS surface.
In this work, PVAm was selected as the polymer matrix, ZIF-8 nanoparticles were used as porous fillers to fabricate mixed matrix composite membrane for H2 purification. First, tannic acid, a water-soluble polymer rich in phenol hydroxyl groups, was added in the synthesis process of ZIF-8 to produce ZIF-8-TA nanoparticles which improved the hydrophilicity and CO2 adsorption of ZIF-8. Then, the compatibility between ZIF-8 particles and PVAm was enhanced by the chemical crosslinking between the quinone groups of oxidized tannic acid and the amino groups in PVAm through Michael addition reaction. Compared with using hydrogen bond force to enhance the compatibility between particles and matrix, the co-valent bond formed by tannic acid grafted on ZIF-8 and PVAm will make the distribution of particles in the film more stable and better compatibility. At the same time, due to the adhesion between TA-Fe3+ complex and PDMS surface, a good compatibility between the hydrophilic separation layer and hydrophobic PDMS gutter layer was achieved. The prepared mixed matrix composite membrane exhibited a good performance for CO2/H2 separation.
Section snippets
Materials
Polysulfone (PSf) ultrafiltration membrane was supplied by Jozzon Membrane Technology Co., Ltd., (China). Polydimethylsiloxane (PDMS) prepolymer was purchased from ShinEtsu (Japan). NVF (vacuum distillation, cryopreservation) and AIBA were bought from Sigma-Aldrich (China). Dimethylimidazole, zinc nitrate hexahydrate, tannic acid and FeCl3 were provided by Aladdin Reagent CO. Ltd. (China). Tetraethyl silicate, dibutyltin dilaurate, hydrochloric acid, ethanol, methanol and n-heptane were from
The effect of TA modification on the structure and morphology of ZIF-8 nanoparticles
As shown in the Fig. 2, the XRD spectra of the prepared ZIF-8-TA showed characteristic diffraction peaks close to that of ZIF-8, which are at 2θ of 7.2°, 10.2°, 12.6°and 17.9°, corresponding to (011), (002), (112) and (222) planes, respectively, consistent with those reported in the literature [44]. For example, the characteristic diffraction peaks of ZIF-8-TA0.1 are at 2θ of 7.28°, 12.7° and 18.02°, indicating that the crystal structure of ZIF-8-TA was very similar to that of ZIF-8. The slight
Conclusions
It has been demonstrated that modification of ZIF-8 with TA was a simple and effective way to enhance its compatibility with hydrophilic polymer matrix. Besides, the strong adhesion of the TA-Fe3+ complex to surface could be taken advantaged to achieve compatibility between hydrophilic separation layer and hydrophobic gutter layer. For the ZIF-8 modification, the optimum TA addition was 0.5 g above which the particles were easy to agglomerate. At the same time, to avoid excessive crosslinking
Acknowledgement
This research is supported by the Natural Science Foundation of China (No.21436009) and the National Key R&D Program of China (No.2017YFB0603400).
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These authors contributed to the work equally and should be regarded as co-first authors.